Its probably blowby. The turbo will always have some oily air in it, lots if your motor is older and driven hard. The boost controller will work by bleeding some air out, with the air comes some oil.
I have heard of people seeing oil all the way up to their boost gauge, which doesnt vent any air, so that is just oil mixing as the air flows back and forth.
the good news is your boost controller wont get rusty!